From the German Offenlegungsschrift 38 33 780, a method is known for the direct synthesis of alkyl glucosides by the acetalization reaction of higher primary alcohols with sugars, particularly with glucose, in which (in abbreviated quotation)
a) mixtures of an aliphatic, primary alcohol, glucose and an acidic catalyst are reacted at an elevated temperature, PA0 b) the molar ratio of glucose to aliphatic alcohol is 1:2 to 1:10, PA0 c) the reaction mixture is kept at an elevated temperature and a reduced pressure until the water of reaction is removed completely, PA0 d) the reaction mixture is cooled to 90.degree. C. and a base is added to neutralize the catalyst and adjust the pH to at least 8, whereupon the normal pressure is restored, PA0 e) the excess alcohol is distilled off gently to a value of less than 5% by weight, and PA0 f) water is added to form a paste and the product is bleached with active oxygen at a pH of 8 to 10. PA0 a) the alcohols can be removed completely; PA0 b) the extraction can be carried out continuously; and countercurrently PA0 c) the extraction agents can be removed completely at low temperatures. PA0 R.sup.1 =a hydrocarbon group with 3 to 21 carbon atoms, PA0 R.sup.2 =CH.sub.2, (C.sub.n H.sub.2n O).sub.m wherein PA0 G=a group derived from an aldose or ketose with 5 or 6 carbon atoms and preferably a group derived from glucose, PA0 a=an average numerical value of 1 to 10 and preferably of 1 to 6.
The German Offenlegungsschrift 39 32 173 relates to a method for the distillative separation of alcohols with chain lengths up to 30 and particularly with chain lengths between 8 and 18 carbon atoms, from a mixture of alkyl glucosides and alcohols not reacted during the synthesis of these alkyl glucosides, the alcohols being removed in two steps, with the distinguishing feature that a falling film evaporator is used in the first step and a thin-layer evaporator in the second step.
Distillative removal of the alcohol imposes an appreciable thermal burden, which is associated with the danger that the product will darken. This goes so far, for example, in the case of stearyl glucoside, that the removal of stearyl alcohol under industrial conditions is practically impossible, without affecting the quality of the stearyl glucoside appreciably. For this reason, various methods for bleaching alkyl glucosides have been described, for example, in EP-OS 0 526 710, the DE-OS 40 19 175 and the DE-OS 39 40 827, in which bleaching is carried out by irradiation, the addition of hydrogen peroxide or the treatment with ozone. At the present time, no method is known, which avoids the critical effect of heat on the crude product, consisting of alkyl glucoside and alcohol, and thus does not lead to the dark coloration.
The purification of sugar surfactants by treatment with carbon dioxide above the critical point has been described in the European patent application 0 404 226. The products, subjected to the extraction, contain esters of a non-reducing sugar and one or several fatty acids. For this method, the extraction must be carried out discontinuously since, at the conclusion of the removal of the fatty acids, the product is present in solid form and the separation process must therefore be interrupted to discharge the end product.